1. The Field of the Invention
The present invention relates to improvements in the process of producing aryl alcohols from aralkyl hydroperoxides, especially to the production phenol from cumene, and most particularly to improving the yields of phenol and reclaimable by-products such as .alpha.-methylstyrene.
2. The State of the Art
Phenol, also known as carbolic acid, is a fundamental starting material for the production of a wide range of organic chemicals. Particularly important are the use of phenol for the manufacture of phenol-formaldehyde resins, and for making bisphenol-A (2,2-bis(4-hydroxyphenyl)propane), which is used in the manufacture of polycarbonate resins (e.g., Lexan.TM.). Phenol is typically produced by either the "cumene process" or the "Raschig process."
The cumene-to-phenol process generally comprises oxidizing cumene, C.sub.6 H.sub.5 --CH(CH.sub.3).sub.2, in air to form cumene hydroperoxide (CHP), C.sub.6 H.sub.5 --C(OOH) (CH.sub.3).sub.2. This hydroperoxide is then cleaved under dilutely acidic conditions to form the phenol product and acetone (2-propanone) as a useful by-product. Acetone and phenol can be used to produce bisphenol-A, which is condensed with phosgene to produce polycarbonate plastics.
In the first step of the cumene-to-phenol process, typically air is bubbled through cumene, at elevated temperatures, with or without the presence of aqueous sodium carbonate, to produce CHP. During this oxidation step, contaminant by-products, particularly .alpha.,.alpha.-dimethylbenzyl alcohol (DMBA; also variously called dimethyl phenyl carbinol, DMPC, or DMFC) and acetophenone (AP), are formed. This oxidation reaction is the primary generator of yield-loss by-products in the cumene-phenol process. There are additional impurities in the products of the oxidation reaction, such as unreacted cumene, and formic and carbonic acids and their salts.
In the cumene-phenol process, dilute acid is used to cleave CHP and produce phenol and acetone. The DMBA contaminant by-product present in the CHP fed to the cleavage reactor dehydrates in the dilutely acidic reaction mixture to form AMS, as just noted. However, the presence in the reaction mixture of the phenol product as well as the acetone, CHP, AP, and water by-products, plus unreacted cumene, allow for further reactions. Those of particular importance are the detrimental reactions of the phenol product with AMS to form cumylphenol and of AMS with itself to form AMS dimer because of their contribution to the yield loss, and the reaction of DMBA with CHP to form dicumyl peroxide (DCP) and water. AMS dimer and cumylphenol are essentially waste by-products from which recovery of useful products is very difficult and expensive; DCP is cleaved by the acid catalyst in the reaction mixture to produce phenol, acetone, AMS.
This cleavage reaction is applicable to aralkyl hydroperoxides in general. More importantly, this type of cleavage reaction is very rapid and very exothermic. (See, e.g., V. G. Jung and G. Just, J. prakt. Chem., 313, 377 (1971), who describe a study of the kinetics of the CHP cleavage reaction using fast flow in small tubes.) To facilitate removing the heat generated in the cleavage reaction and thereby control the reaction, commercial plants use continuously-stirred tank reactors (CSTRs) operating with acetone reflux. The reaction medium comprises mostly the reaction products, phenol and acetone, with only minimal concentrations (&lt;1%) of CHP. The reaction mixture also contains significant amounts of the contaminant by-products and the various cleavage reaction by-products.
U.S. Pat. No. 4,358,618 teaches a variation on the cumene process whereby the formation of DCP as an intermediate is promoted in a first reaction step, and then the DCP is decomposed in a second reaction step to yield phenol, acetone, and AMS. The residence times and reaction temperatures are chosen to balance the maximization of DCP in the first step and its subsequent decomposition to phenol and AMS. It is also suggested that a third, small tubular reactor be used between the two steps to promote the decomposition of CHP remaining after the first reaction step.
U.S. Pat. No. 4,173,587 discloses the use of a rhenium-containing catalyst for high selectivity in producing phenol and acetone from the cleavage of CHP and subsequently recovering DMBA by distillation. Reaction times are between ten and 25 minutes. Also, the AMS derived in this process appears to be derived from the CHP as a by-product of the rhenium-catalyzed reaction, rather than derived from the dehydration DMBA formed during oxidation of cumene as for the general process noted above.
U.S. Pat. No. 4,016,213 varies the typical cumene process by immediately neutralizing the reaction product to prevent the dehydration of DMBA to AMS.
U.S. Pat. No. 4,207,264 is particularly concerned with the formation of highly colored by-products, of concern typically in the production of hydroquinone (1,4-para-dihydroxy benzene) from p-diisopropylbenzene dihydroperoxide; hydroquinone is used as a rubber antioxidant. To avoid these by-products, the cleavage reaction is conducted in a tubular reactor to prevent any portion of the reaction mixture from remaining in the reaction zone longer than the desired average residence time. Residence times of one to 15 min., preferably five to 12 min., are taught.
P. L. Beltrame et al., "Side Reactions in the Phenol/Acetone Process. A Kinetic Study," Ind. Eng. Chem. Res., 27, 4-7, 1988, discuss the reaction of dimethylphenylcarbinol (DMFC) in the presence of phenol, acetone, and sulfuric acid to give AMS and other byproducts, including diphenyl-substituted pentenes, cumylphenol, and phenyl cumyl ether. The kinetics are derived using a reaction scheme having a carbocation intermediate. They teach that phenol assists the acid-catalysis of various reactions and thus the total acidity of the reaction mixture (sulfuric acid plus phenol) strongly effects the reaction time leading to the maximum AMS yield.
U.S. Pat. No. 4,310,712 discloses a process of producing phenol, acetone and AMS from cumene hydroperoxide using a sulfuric acid catalyst in a reactor without significant back-mixing (i.e., with plug-flow) and controlling the reaction temperature by evaporation of acetone from the reaction mixture. In contrast, U.S.S.R. Pat. No. 851,851 discloses decomposing cumene hydroperoxide in a multisection reactor and adding cold acetone to each section to stabilize the reaction temperature. These contrasting disclosures illustrate how important the type of reactor and reaction rate can be to controlling this process.